In high performance liquid chromatography (HPLC), piston pumps are customarily used for conveying liquids. The piston can be formed here from a smooth rod manufactured within highly exacting tolerances, for example made from zirconium oxide or sapphire. Said rod moves relative to a stationary seal, past which the rod slides with as little friction as possible. The seal is intended to close the gap between a pump head forming the cylinder and the piston oscillating therein. There are two functional sealing regions in this case: firstly between the positionally fixed seal and the moving piston and secondly between the positionally fixed seal and the likewise positionally fixed pump head.
There are also very similar conditions in HPLC metering syringes which provide a precisely defined quantity of the medium to be analyzed. If the stroke space of the metering syringe is joined into the conveying path of the high pressure pump, the stroke space of the syringe correspondingly also has to be readily sealed at the plunger in order to permit the high operating pressures of frequently more than 1500 bar. For high pressure pumps or metering syringes in HPLC technology, there are therefore comparable requirements with regard to the sealing.
For metering syringes, the axial movability of known seals has proven problematic. The reciprocating movement of the piston leads to migration of the seal at only slightly increased operating pressures. Since metering syringes specifically depend on a very precise determination of the volume received, even only a slight displacement of the seal makes it difficult to reproduce the sample quantities to be received.
Furthermore, there is the problem that the known seals have to resolve dynamic and static sealing tasks, for which, however, different material properties are generally required. Therefore, known high pressure seals are customarily composed of thermoplastic material with good sliding properties, but low rigidity. For high pressure applications, seals having sealing lips which also utilize the system pressure for sealing purposes are known. The sealing lips which are spaced apart by a recess bear here radially on the inside against the shaft to be sealed or the piston rod, while the outer sealing lip, which is connected integrally to the inner sealing lip, bears against the housing. The cavity formed between the sealing lips is subjected to system pressure in order to push the inner sealing lip radially inward for an improved sealing action (hydraulic seal). Since, however, such a seal does not function adequately for operating pressures only just above ambient pressure, the seals are frequently prestressed.
However, the dynamically stressed sealing lip has to have sliding properties which are as good as possible, but such a material is frequently too flexible to be inherently prestressed. A metallic spring element is therefore frequently inserted into the cavity of the seal in order thereby to push the sealing lips radially apart against the housing or the piston rod or shaft. However, in HPLC technology, metals along the conveying path of the sample are undesirable because of possible chemical reactions with the sample. In addition, the cavity for the metallic prestressing element is a dead space which is difficult to flush through and in which residues or air bubbles may accumulate, which should be avoided. In general, in HPLC technology, there is a trend toward reducing the flow-through volume and toward metal-free paths for a biocompatible application. However, even non-metallic prestressing elements, for example O rings, are generally not chemically inert to all of the substances to be expected in the medium. They may be attacked and destroyed by said substances such that the sealing action is no longer provided.
For the pure static sealing, good sliding properties of the sealing material are not required since the components to be sealed in relation to one another do not carry out any relative movement. The integral seals previously known from the prior art do not meet these different requirements regarding the sealing properties.